Phase Change Compositions

ABSTRACT

There are herein described phase change materials containing sodium acetate trihydrate having improved homogeneity, a process for the preparation of said materials, and their utility in phase change systems. More particularly, the present invention relates to the use of phase change compositions comprising sodium acetate trihydrate, at least one alkali soluble polymer for inhibition of sodium acetate anhydrous crystal formation in sodium acetatetrihydrate containing phase change materials, and at least one sodium acetate trihydrate nucleation promoter, and, if a lower phase change temperature is required, at least one melting point depressing agent.

FIELD OF THE INVENTION

The present invention relates to phase change materials containingsodium acetate trihydrate having improved homogeneity, a process for thepreparation of said materials, and their utility in phase changesystems. More particularly, the present invention relates to the use ofphase change compositions comprising sodium acetate trihydrate, at leastone alkali soluble polymer for inhibition of sodium acetate anhydrouscrystal formation in sodium acetate trihydrate containing phase changematerials, and at least one sodium acetate trihydrate nucleationpromoter.

BACKGROUND OF THE INVENTION

There are many heating and cooling systems on the market and many ofthese are reliant on fossil fuels. With the ever increasing demand formore environmentally friendly systems various alternative systems basedon sunlight or water have been proposed such as for examplephotovoltaics, solar thermal electricity generators, hydroelectricity,wave power and bio-fuels.

An issue common to all solar-driven renewable energy conversion devices,some hydro-driven devices, and wind turbines is that they cannot operate“on demand”, as the sun does not always shine, the seas are not alwayshigh and the wind does not always blow. This means that at some timesthese so-called intermittent renewable sources will generate electricitywhich cannot be easily integrated into their corresponding localelectricity grids, and as such there have been a number of storagesolutions proposed.

The thermal energy storage system, proposed in WO 2009/138771 convertssurplus electrical energy from intermittent renewable sources into heator cool when available, store the so-converted heat or cool in a thermalstore, and then make it available as useful heat or cool on demand usingphase change materials (PCMs) to effect the energy conversion via theirinherent solid-liquid phase changing properties.

For practical application in domestic situations phase change materialscapable of supplying warm, or even hot, water that is just above thecomfort level temperature of the individual requiring the heated waterare needed. In addition, phase change materials suitable for suchpractical utility should achieve rates of heat transfer into and out of,their phase changes, which are commensurate with domestic use, as wellas deliver acceptable levels of thermodynamic stability (efficiency).

Sodium acetate trihydrate, (SAT), has a solid-liquid phase change withinthe desired temperature range for domestic utility. However, thepractical applications of SAT as a phase change material (PCM) arelimited by the unique and incongruous manner in which it melts, goingfrom solid SAT, to a mixture of liquid SAT and a solid (sodiumacetate/SA) at a set temperature, 58° C. This inability to provide afully liquid solution at 58° C. is an issue which is reflected in thethermodynamic stability of aqueous solutions upon initial heating, aswell as the thermodynamic stability of, re-formed, aqueous solutionsprovided following re-heating after cooling in accordance with thenormal heating/cooling cycles in phase change systems.

This solid-formation on melting is a problem for use as SAT as a PCMbecause this solid SA, once formed, would ordinarily be retainedthroughout the lifetime of the PCM in a phase change system.

Previous attempts to overcome this issue via the utility of threedimensional cross-linked polymers, such as cellulose based polymers andsuper absorbent polymers, to act as solid supports have beenunsuccessful because, independent of any apparent initial reduction ofsolid formation, the underlying problem of solid sodium acetateformation is unresolved, and over time solid sodium acetate will stillprecipitate out of solution, and collect at the base of the PCM storagevessel irreversibly. For utility as a PCM, no solution which has afinite lifetime is acceptable.

It is an object of at least one aspect of the present invention toobviate or mitigate at least one or more of the aforementioned problemsin relation to the utility of sodium acetate trihydrate as a potentialPCM for use in aqueous phase change systems.

It is an object of at least one aspect of the present invention toprovide improved phase change materials containing sodium acetatetrihydrate having desirable homogeneity; resistance to SA formation,which are suitable for use in phase change systems.

It is an object of at least one aspect of the present invention toprovide improved phase change materials containing sodium acetatetrihydrate which can be heated, cooled and re-heated in repeated cycleswith retention of thermodynamic stability.

The Applicant has developed novel and inventive aqueous compositions foruse as phase change materials comprising: sodium acetate trihydrate; atleast one alkali soluble polymer for inhibition of sodium acetateanhydrous crystal formation in sodium acetate trihydrate containingphase change materials; and at least one sodium acetate trihydratenucleation promoter. The Applicant has also developed a process for thepreparation of said improved phase change materials.

SUMMARY OF THE INVENTION

The Applicant has developed novel and inventive phase changecompositions containing sodium acetate trihydrate as a phase changematerial.

Accordingly the present invention provides compositions containingsodium acetate trihydrate as a phase change material comprising:

-   -   (a) sodium acetate trihydrate or sodium acetate anhydrous;    -   (b) at least one suitable alkali soluble polymer;    -   (c) at least one suitable nucleation promoter; and    -   (d) water.

As demonstrated in the Examples hereinafter the Applicant hassurprisingly found that the compositions of the present invention whichcontain sodium acetate trihydrate as a phase change material displayunprecedented improvements in homogeneity and thermodynamic stabilityproperties than previously achievable using sodium acetate trihydrate asa PCM. In particular the Applicant has found that the compositions ofthe present invention which contain sodium acetate trihydrate as a phasechange material are resistant to the nucleation of crystalline sodiumacetate on heating and cooling.

According to a further aspect the present invention provides for the useof the compositions according to the invention as phase change materialssuitable for use in phase change systems.

The Applicant has also developed a novel process for the preparation ofthe compositions according to the invention. According to a furtheraspect the present invention provides a process for the preparation ofcompositions containing sodium acetate trihydrate as a phase changematerial which comprises:

-   -   (a) mixing an aqueous solution comprising sodium acetate        anhydrous with at least one suitable alkali soluble polymer; and        at least one nucleation promoter;    -   (b) heating the resultant mixture to provide a 58° C. phase        change material containing sodium acetate trihydrate.        When sodium acetate trihydrate is used, the following steps are        followed:    -   (a) heating the sodium acetate trihydrate to a temperature above        58° C.; and    -   (b) mixing the sodium acetate trihydrate with at least one        suitable alkali soluble polymer; and at least one nucleation        promoter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings:

FIG. 1 is a phase diagram of sodium acetate and water, showing thesolubility limit of sodium acetate in water, at about 58° C., is atabout 58.0%, whereas the corresponding sodium acetate trihydrate, whichmelts at 58° C., is made up of 60.28% of SA and 39.72% of water, a valuenoticeable higher than the solubility limit at 58° C., as indicated bypoint 2;

FIG. 2 is a proposed mechanism where the polymer interacts with thesurface of highly metastable sub-critical clusters of sodium acetatemolecules to prevent, or block, their further growth into crystallites,and hence precipitated material in due course, via a blocking mechanism;and

FIG. 3 is a representation of crystallisation temperatures obtained byadding varying levels of acetamide.

DETAILED DESCRIPTION

The novel compositions according to the present invention are aqueouscompositions containing sodium acetate trihydrate, NaOAc·3H₂O, alsoknown as SAT, as a phase change material. Any form of sodium acetateanhydrous, also known as SA, may be used in the preparation the novelcompositions containing SAT as a PCM in accordance with the invention.For the avoidance of doubt this means that as all solid forms of NaOAcare crystalline, and therefore any crystalline form may be used.

The level of sodium acetate anhydrous used in the preparation of theaqueous compositions according to the invention is between about 40% toabout 60% by weight of the total composition. This corresponds to arelative amount of from about 66% to about 100% of sodium acetatetrihydrate in the aqueous compositions of the invention. For theavoidance of doubt the compositions according to the present inventionare substantially free-from, and are more preferably free-from solidanhydrous sodium acetate.

For the avoidance of doubt, where the actual or relative amounts ofwater in any particular composition according to the invention describedherein are not specified it should be understood that the actual orrelative amount of water required will be that sufficient to reacheither 100% of composition, either by weight or relative volume. Watermay be used either in purified, or distilled form, or from regularsupply.

As indicated hereinbefore, despite having a phase change within thedesired temperature range for domestic heating purposes, the utility ofsodium acetate trihydrate as a potential PCM to-date has been hampereddue to inherent melting incongruences. During the melting process sodiumacetate trihydrate changes from solid SAT, to a mixture of liquid SATand solid at a set temperature, 58° C. This is due to the formation ofsodium acetate anhydrous, NaOAc, or SA. For the avoidance of doubt,where the term sodium acetate, or SA, is utilised herein it means sodiumacetate anhydrous, as opposed to any hydrated form, or the trihydrate,SAT in particular. As indicated by point 1, in FIG. 1, a phase diagramof sodium acetate and water, the solubility limit of sodium acetate inwater, at about 58° C., is at about 58.0%, whereas the correspondingsodium acetate trihydrate, which melts at 58° C., is made up of 60.28%of SA and 39.72% of water, a value noticeable higher than the solubilitylimit at 58° C., as indicated by point 2, in FIG. 1. When SA is formedduring the melting of aqueous SAT, a fully liquid state may bere-instated via the addition of more water, to alter the composition tothe composition of 58% SA, and solubilise SA anhydrous, therebyproviding a fully liquid material at 58° C. The so-produced solution isthermodynamically stable, i.e. it is neither in the metastable zone, norin the supersaturated zone, thus no further solid material (SA) shouldcrystallise out.

As explained hereinbefore, such a re-instated liquid solution comprisingwater and sodium acetate trihydrate is not suitable for use as a phasechange agent because, it is not capable of being cooled and reheated toprovide a thermodynamically stable homogeneous liquid. As demonstratedin the Examples hereinafter, the Applicant has found when such asolution is cooled down and seeded with some sodium acetate trihydrate,a solid sample of sodium acetate trihydrate is formed. Applicant hasalso found that when such a cooled solution is heated up to 58° C.again, a solution with some solid sodium acetate anhydrous, SA, isformed i.e. the solution is not in fact in thermodynamic equilibrium.

Mixing and/or agitation would return the composition to itsthermodynamic equilibrium, a homogenous solution; however without suchmechanism, as in this invention, a homogenous solution does not prevail.

As illustrated in FIG. 1, crystalline sodium acetate trihydrate melts toform sodium acetate anhydrous and a concentrated solution of aqueoussodium acetate. In simple terms, and in accordance with general chemicalpractice, the addition of more water added would be expected to dissolvethis additional sodium acetate (the anhydrous solid). More accurately,this extra water would be expected to dilute the concentrated solutioncomprising sodium acetate anhydrous and water, which in turn wouldenable the solid sodium acetate anhydrous to dissolve into it. Mostsurprisingly, the Applicant has demonstrated that this does not occur inpractice, and further that the resistance to dissolution of theanhydrous solid is in fact unaltered due to the additional water level.

Without wishing to be bound to any particular theory it is postulatedherein that, during the melting process, molecules of sodium acetatetrihydrate dissociate incongruently, and also that there is rapidexchange between the sodium acetate in the liquid phase and the sodiumacetate in the solid phase. The solid sodium acetate anhydrous moleculesare thought to initially be present in small clusters. Such smallclusters are thought to possibly contain hundreds of molecules of SA,and are not considered to be crystalline. It is further proposed hereinthat as any one of these clusters increases in size it can eventuallyattain the critical cluster size, or threshold size, required to becomea crystallite. It is further proposed herein that such crystallites,grouping together, form the dense crystals of sodium acetate that areobserved as the unwanted solid precipitated material in theaforementioned Example. The Applicant has observed that the formation ofsolid precipitate, during the melting process, occurs too quickly forthe dissolution of additional water to have any significant impact.

The Applicant has resolved this issue of non-homogeneous liquidformation in aqueous phase change materials containing sodium acetatetrihydrate via the provision of PCMs containing NaOAc·3H₂O which areresistant to the formation of NaOAc crystallites via the utility of oneor more particular alkali soluble polymers.

Without being bound to any particular theory it is thought that theparticular polymers utilised in the PCMs containing sodium acetatetrihydrate according to the present invention composition provideresistance to the formation of precipitated NaOAc in aqueous NaOAc·3H₂Osolutions via a combination of effects including: viscosity effects;crystal habit modifying behaviour; 3D-lattice effects. As such, the oneor more alkali soluble polymers suitable for use herein may also bereferred to as SA crystal inhibitors, or inhibitors of the formation ofcrystalline SA from aqueous solutions.

At one level by harnessing the ability of these particular polymers toincrease the viscosity of solutions it is believed that at least some ofthe sodium acetate anhydrous formed during the melting process mayremain suspended long enough to dissolve and thereby reduce thepotential for SA precipitation from solution, and subsequent collectionwithin the phase change system equipment during use. It is also proposedherein that use of these particular polymers would provide solid sodiumacetate having a significantly increased effective surface area insolution, versus that previously possible (for SA only aqueous systems)when the solid is amassed at the base and only the top of this solidlayer is in contact with the solution. It is further proposed hereinthat even dispersal of the solid sodium acetate in this more viscoussolution for an extended period of time, may increase the dissolutionrate and thereby reduce the rate at which the anhydrous form of sodiumacetate could form, and potentially may prevent formation altogether.

Polymers suitable for use in the novel compositions herein containingSAT as PCMs in accordance with the first or further aspects of theinvention as detailed herein are soluble in aqueous alkaline solution.As defined herein polymers suitable for use herein are soluble inaqueous alkaline solutions having a pH in excess of about pH 8. Moreparticularly polymers suitable for use herein are soluble in highlyalkaline aqueous solution having a pH of about pH 9, such as aqueoussodium acetate solution. The compositions herein include one or morepolymers, as defined herein, wherein each polymer may be independentlypresent at levels of from about 0.1% to about 10%, from about 0.2% toabout 4%, from about 0.5% to about 2% are utilised in the aqueouscompositions according to the present invention.

In addition, polymers suitable for use herein have one or morecarboxylic acid groups and may be utilised as acids, or as acid salts.For the avoidance of doubt the term polymer as used herein includes bothpolymers of repeated singular monomeric units, and co-polymers comprisedof mixed monomeric units having varying repeating patterns.

A group of preferred polymers for use herein are polymers havingrepeating units of general formula I:

−{[X] _(n) −[Y] _(m)}_(z)−  I

and salts thereofwherein z is 10 to 1,000;n=1 to 10 to 1,000; and wherein m=0 to 1,000

wherein the ratio of n:m is in the range of from about X: Y and whereinthe molecular weight of the polymer is in the range of from about P toabout Q,

X is independently selected from groups based on the following monomers:ethylene, acrylic acid, methyl methacrylate, acrylamide, ethylmethacrylate, ethacrylic acid, ethyl oxide, diallyldimethylammoniumchloride, vinyl pyrrolidone, N-isopropylacrylamide, styrene, maleic acidand mixtures thereof,

Y is independently selected from groups based on the following monomers:ethylene, acrylic acid, methyl methacrylate, acrylamide, ethylmethacrylate, ethacrylic acid, ethyl oxide, diallyldimethylammoniumchloride, vinyl pyrrolidone, N-isopropylacrylamide, styrene, maleic acidand mixtures thereof,

A group of preferred polymers according to general formula I for useherein have monomeric repeating units of general formula II.

and salts thereof wherein n, m and z are as defined hereinbefore

wherein the ratio of n:m is about 1:2 and wherein the molecular weightis in the range of from about 1,000 to 1,000,000.

Preferred polymers of general formula II for use herein are either acidsor acid salts, more particularly either acids or metal acid salts, andespecially acids or sodium, potassium or zinc acid salts. The copolymerof methacrylic acid and methyl methacrylate, poly(methacrylic acidco-methyl methacrylate) having a molecular weight of from about 500,000to about 1,000,000, and the sodium, potassium or zinc salts thereof arepolymers within general formulae I, II and III, and are particularlysuited for use herein.

Another group of preferred polymers for use herein according to generalformula I for use herein have repeating units of general formula III:

and metal salts thereof selected from sodium or potassium, and

wherein z and n are as defined herein before.

A preferred polymer of formula III for use herein is poly(methacrylicacid), particularly the sodium salt, preferably in 30 or 40% aqueoussolution wherein the polymer has an average molecular weight of about10,000 or 4-6,000

Thus, the present invention provides compositions as definedhereinbefore wherein the one or more alkaline soluble polymers areselected from general formulae II or III and mixtures thereof eachindependently present at a level of from about at levels of from about0.1% to about 10%, from about 0.2% to about 4%, from about 0.5% to about2%. In addition, there are provided compositions as defined hereinbeforewherein the polymer is: the copolymer of methacrylic acid and methylmethacrylate, poly(methacrylic acid co-methyl methacrylate) having amolecular weight of from about 500,000 to about 1,000,000, or a sodium,potassium or zinc salts thereof; the poly(methacrylic acid), or thesodium salt having an average molecular weight of about 10,000, andwherein either polymer may be independently present at a level of fromabout at levels of from about 0.1% to about 10%, from about 0.2% toabout 4%, from about 0.5% to about 2%. According to a further aspect thepresent invention provides a composition having either one or the abovetwo polymers at the levels defined above.

The improved PCM compositions according to the present inventioncontaining SAT as a PCM have been demonstrated to form homogeneousliquids without the need to add excessive additional water, which is instark contrast to the precipitated systems observed without polymers.However it is the demonstrated improvements in both homogeneity andthermodynamic stability following cooling and re-heating, between theSAT containing PCM systems of the invention with polymers, when comparedto the behaviour displayed by sodium acetate trihydrate without polymerswhich are truly revolutionary.

Without being bound to any particular theory it is proposed herein thatphenomenal abilities observed for the various different SAT/polymersystems tested for the delivery unprecedented inhibition of nucleationin aqueous sodium acetate trihydrate systems is due to a consistent,underlying technical effect common to them all. Whilst the precisemechanism of crystal nucleation inhibition in these systems is unknown,it is proposed herein that the polymer interacts with the surface ofhighly metastable sub-critical clusters of sodium acetate molecules toprevent, or block, their further growth into crystallites, and henceprecipitated material in due course, via a blocking mechanism. FIG. 2provides a proposed illustration of this mechanism.

The Applicant has found that even when the polymeric systems herein areseeded with SA, to in effect force the formation of solid material, theso-formed solid material is very different in both appearance andbehaviour to that observed in the corresponding solid material formed innon-polymeric systems. It is in this impact upon the behaviour of theresultant solid, which suggests that the solid may exist in a differentstructural habit, and that the polymers act as crystal habit modifiers.

Whilst these modified polymer containing aqueous solutions of sodiumacetate are indeed most surprisingly and desirably resistant to SAformation, they are not suitable for utility as PCMs because, as hasbeen most clearly demonstrated both by the experiments and resultsdiscussed herein and as the SA phase diagram of FIG. 1, sodium acetatetrihydrate does not readily nucleate, even in supersaturated solutions.This means that a nucleating agent is required to promote the nucleationof the sodium acetate trihydrate from the aqueous solution.

Nucleating agents, as defined herein are also known as nucleators, ornucleation promoters. In some cases, the effective pairing of aparticular material with a particular nucleating agent can be the resultof iso-structural similarities between the nucleator and the salthydrate in their crystalline forms, and indeed this is the case withmany known-pairs. For example strontium chloride hexahydrate acts as anucleator for calcium chloride hexahydrate, and sodium tetraboratedecahydrate acts as a nucleator for sodium sulfate decahydrate and ineach pairing they have very similar molecular packing in theirrespective crystalline forms.

For sodium acetate trihydrate identification of a suitable nucleator isa more challenging matter. Whilst disodium hydrogen phosphate (DSP) andtetrasodium pyrophosphate (TSPP) have been identified as potentialnucleators for SAT their mechanism of action remains unknown. (T Wadaand R Yamamoto “Studies on salt hydrates for latent heat storage. 1.Crystal nucleation of sodium acetate trihydrate catalyzed by tetrasodiumpyrophosphate decahydrate, Bulletin of the Chemical Society of Japan.Volume 55, page 3603, 1982; T Wada, R Yamamoto and Y Matsuo “Heatstorage capacity of sodium acetate trihydrate during thermal cycling”,Solar Energy. Volume 33, pages 373 to 375, 1984; and H Kimura,“Nucleating agents for sodium acetate trihydrate”, Journal of theJapanese Association of Crystal Growth. Volume 9, issue 3, page 73,1982.)

Furthermore there is an acknowledged so-called deactivation of thesenucleators at high temperatures which undermines their potential forconsideration as potential pairs for SAT PCMs for use in phase changesystems because by their nature PCMs are intended for long-term use, andrequire on-demand, reliable activation throughout the anticipatedheating/cooling/re-heating cycles therefor.

Surprisingly the Applicant has identified a particular hydrate ofdisodium hydrogen phosphate, the dihydrate, to be the active nucleatingspecies for SAT, and has also demonstrated that utility of this hydratein aqueous polymeric solutions containing SAT as detailed hereinbeforeprovides compositions highly suited for use as PCMs in phase changesystems. The compositions according to the present invention typicallycontain one or more nucleation promoters each independently present at alevel of from about 0.1% to about 5%, from about 0.2% to about 3%, fromabout 0.5% to about 2%.

Thus the present invention additionally provides compositions containingsodium acetate trihydrate as a phase change material comprising:

-   -   (a) from about 48 to about 60% of sodium acetate anhydrous;    -   (b) from about 0.1% to about 10% of at least one suitable alkali        soluble polymer;    -   (c) from about 0.1% to about 5% of at least one suitable        nucleation promoter; and    -   (d) water to balance.

Alternatively, when sodium acetate trihydrate is used, the compositionabove changes to:

-   -   (a) from about 80% to about 100% of sodium acetate trihydrate;    -   (b) from about 0.1% to about 10% of at least one suitable alkali        soluble polymer;    -   (c) from about 0.1% to about 5% of at least one suitable        nucleation promoter; and    -   (d) water to balance.

Whilst any material capable of nucleating SAT is suitable for useherein, preferred materials which both nucleate SAT and retain theireffectiveness at high temperatures are particularly suited for use inthe compositions for use as PCMs according to the present invention.Such materials include: disodium hydrogen phosphate (DSP); tetrasodiumpyrophosphate (TSPP); and hydrated forms thereof. Particular materialssuitable for use herein are disodium hydrogen phosphate dihydrate, andtetrasodium pyrophosphate decahydrate. Thus, the present inventionprovides compositions as defined hereinbefore wherein the nucleationpromoters are disodium hydrogen phosphate (DSP); tetrasodiumpyrophosphate (TSPP); and hydrated forms thereof each independentlypresent at a level of from about 0.1% to about 5%, from about 0.2% toabout 3%, from about 0.5% to about 2%. In addition, there are providedcompositions as defined hereinbefore wherein the nucleation promotersare disodium hydrogen phosphate dihydrate and tetrasodium pyrophosphatedecahydrate and wherein the total level of these promoters is from about0.5% to about 5%, from about 0.2% to about 2.5%, from about 0.5% toabout 2%.

In addition to the polymer and the nucleation promoter the compositionsaccording to the invention may additionally comprise a further agent tomodify the melting point of SAT when in use as a PCM. Any suitable agentcapable of providing a desirable melting point modification can be used,for the avoidance of doubt melting point modification, means a loweringof the melting and crystallisation point. Such modifying agents may beutilised at a relative concentration level of from about 1% to about25%, 5% to about 25%, from about 10% to about 20%, from about 2% toabout 10% of the total mass. Example agents for lowering the meltingpoint of SAT in the compositions herein include: metal salts such asithium acetate dihydrate; and organic compounds such as acetamide andtrimethylolethane which could also include non-metal salts, e.g.ammonium acetate.

Thus the present invention additionally provides compositions containingsodium acetate trihydrate as a phase change material comprising:

-   -   (a) from about 35% to about 60% of sodium acetate anhydrous;    -   (b) from about 0.1% to about 10% of at least one suitable alkali        soluble polymer;    -   (c) from about 0.1% to about 5% of at least one suitable        nucleation promoter;    -   (d) from about 1% to about 95% of an optional melting point        depressing agent; and    -   (e) water to balance.

Preferred herein are compositions wherein the lithium acetate dihydrateis utilised as a melting point depressing agent at levels of from about1% to about 25%, from about 5% to about 25%, from about 10% to about20%.

Lithium acetate dihydrate is 64.67% LiOAc and 35.33% water, values belowcopied from above but x by 64,67.

Preferred herein are compositions wherein the lithium acetate anhydrousis utilised as a melting point depressing agent at levels of from about0.65% to about 16.17%, from about 3.23% to about 16.17%, from about6.47% to about 12.93%.

The melting point may also be depressed by addition of acetamide andtrimethylolethane.

The compositions of the invention containing SAT as a PCM can be madeaccording to a process comprising:

-   -   (a) mixing an aqueous solution comprising sodium acetate        anhydrous with at least one suitable alkali soluble polymer; at        least one nucleation promoter, and at least one melting point        depressing agent;    -   (b) heating the resultant mixture to provide a 58° C. phase        change material containing sodium acetate trihydrate.

According to a preferred process herein the from 35% to about 60% ofsodium acetate anhydrous, from about 0.1% to about 10% in total ofsuitable alkali soluble polymer(s), from about 0.1% to about 5% in totalof one or more nucleation promoter are utilised and optionally fromabout 1% to about 25% of melting point depressing agent may be includedin the mixing stage. An example of this process is found in Example 3hereinafter.

According to a preferred process herein the from 35% to about 60% ofsodium acetate anhydrous, from about 0.1% to about 10% in total ofsuitable alkali soluble polymer(s), from about 0.1% to about 5% in totalof one or more nucleation promoter are utilised and optionally fromabout 1% to about 95% of melting point depressing agent may be includedin the mixing stage.

Alternatively the polymer may be formed in situ in the molten (heated)aqueous SAT mixture, in which instance the relevant monomers would beadded into the aqueous solution and polymerisation would be initiated.Therefore, the PCM can be made in either the heat battery enclosurebefore including the heat exchanger and sealing it or the PCM can bemade in an external vessel and then poured into the enclosure containingheat exchanger.

The following non-limiting examples provided in the Experimental resultshereinafter are representative of the PCM compositions according to theinvention, as are the processes for their preparation.

Experimental Results EXAMPLE 1 Addition of Polymer 1 to Aqueous SodiumAcetate to Prevent Precipitation

1% (2g) of a poly(methacrylic acid co-methyl methacrylate) 2:1 copolymerhaving a molecular weight of 500,000-1,000,000 (available from Fluka, asPolyacrylic acid, cas 25086-15-1 (labelled medium viscosity, mr˜500,000-1,000,000, copolymer of methacrylic acid and methylmethacralate) was added with stirring to a well-mixed 198g aqueoussolution of sodium acetate (available from VWR International Ltd. (UK)as sodium acetate anhydrous 99%) at a concentration of 58.24% (17.002mol dm⁻³) at from about 60 to about 70° C. and then allowed to cool downto room temperature (RT).

At RT a homogenous liquid was observed which is in stark contrast to thesame solution but without any polymer where significant of precipitationof sodium acetate was observed. Further, cycling experiments werepreformed throughout a temperature range of between about 25 to about80° C. which confirmed that the beneficial effects observed forpolymer-assisted solution were consistent across the range tested. Inaddition, further experiments confirmed that sodium acetate could onlybe initiated to form with seeding at RT.

EXAMPLE 2 Addition of Polymer 2 to Aqueous Sodium Acetate to PreventPrecipitation

(0.66% with respect to only polymer) 22.6 g of a 30% aqueous solution ofpoly(methacrylic acid, sodium salt) having a molecular weight of 9,500(available from Sigma Adrich, UK, as Poly(methacrylic acid, sodium salt)solution average M_(n)˜5,400, average M_(w)˜9,500 by GPC, 30 wt. % inH₂O, cas 54193-36-1) was added to a 59.16% (17.66 mol dm⁻³) aqueoussolution of sodium acetate(1,000 g) in accordance with the method ofExample 1 and then allowed to cool down to room temperature (RT).

EXAMPLE 3 Qualification that Polymer Systems Provide Homogeneous Liquidsat 58° C.

Test samples containing various levels of polymers 1 and 2, and varyinginitial aqueous concentrations of sodium acetate, were left undisturbedat RT for several weeks. Where sodium acetate were observed, it appearedas a large white mass taking up the whole sample container. This mass isthought to be an intricate mixture of sodium acetate and water, and yethad the appearance of a solid. When the sample container was squeezedthe mass was observed to be very soft. On careful stirring this mass wasfound to appear as fine needle-like crystals and furthermore, heatingsuch samples to 58° C. resulted in homogenous liquids, which were inagreement with the phase diagram of FIG. 1, point 2, as discussedhereinbefore.

In addition test samples were carefully seeded with sodium acetate werealso left undisturbed, and the resulting solid sodium acetate appearedas a large white mass taking up the whole sample container. This mass isthought to be an intricate mixture of sodium acetate and water, and yethad the appearance of a solid. When the sample container was squeezedthe mass was observed to be very soft. On careful stirring this mass wasalso found to appear as fine needle-like crystals, and furthermore, onheating to 58° C., these samples also provided in homogenous liquids,which were in agreement with the phase diagram of FIG. 1, point 2.

This ability to form homogeneous liquids without the need to addexcessive water is in stark contrast to the precipitated systemsobserved without polymers. However the observed improvements in bothhomogeneity and thermodynamic stability following cooling andre-heating, between the systems of the invention with polymers and whencompared to sodium acetate trihydrate without polymers is trulyrevolutionary.

EXAMPLE 4 Identification of Active Nucleator for SAT in DSP

Variable temperature powder X-ray diffraction was utilised to identifythe dihydrate of disodium hydrogen phosphate as the active nucleator.Samples of sodium acetate trihydrate with disodium hydrogen phosphatedihydrate were cooled and crystallisation of sodium acetate trihydratetook place. Heating the sample to 90° C. resulted in the transition fromthe dihydrate to anhydrous disodium hydrogen phosphate. Subsequentcooling of this sample, now containing SAT and anhydrous DSP to 25° C.,did not return the anhydrous DSP to the DSP dihydrate, and thus nocrystallisation of sodium acetate trihydrate could take place.

Additional experiments were carried out to confirm that no deactivationof this system occurs at the temperatures required for utility incompositions for use as PCMs, via either seeding with sodium acetatetrihydrate or the active nucleators. The results of these experimentshave confirmed that upon such seeding “reactivation” of the nucleatorsoccurs and their effectiveness is fully restored. This is in agreementwith previous observations but the Applicant is the first to understandand characterise the hitherto unexplained phenomena.

EXAMPLE 5 Preparation of PCM Formulations Containing SAT

PCM 1 PCM 2 Material Weight % Weight % Polymer 0.67% 0.67% Sodiumacetate 56.20% 44.45%  Water 40.31% 31.87%  DSP•2H₂O 1.20% 1.29%TSPP•10H₂O 1.62% 1.73% LiOAc•2H₂O —  20%

Formulation 1 was prepared as follows sodium acetate anhydrous (134.1kg, 1634.77 mol), water (94.6 kg, 5251.18 mol) and a polymethacrylicacid polymer (mol wt 9,500) as a 30% aqueous solution (5.3 kg), DSP (2.3kg, 16.20 mol) and TSPP (2.3 kg, 8.65 mol) were mixed together andheated to about 70° C. This resultant 58° C. PCM produced via thisprocess has no anhydrous sodium acetate formation.

Formulation 2 was prepared as for Formulation 1. The resultant 50°PCMproduced also had no anhydrous sodium acetate formation.

Thus the PCMs according to the present invention are free from SA whenviewed by the naked eye and experimental techniques such as X-ray powderdiffraction.

EXAMPLE 6 Addition of Acetamide to Sodium Acetate Trihydrate to DepressMelting Point

Ratio of SAT to acetamide needed for mixes

Mass (g) Molar sodium acetate Ratio trihydrate acetamide 9:1 19.07980.9202 8:2 18.0421 1.9579 7:3 16.8629 3.1371 6:4 15.5112 4.4888 5:513.9462 6.0538 4:6 12.1130 7.8870 3:7 9.93611 10.0639 2:8 7.3091 12.69091:9 4.0760 15.9240a) Starting from the trihydrate form of sodium acetate:

95.53 wt % sodium acetate trihydrate (CAS 6131-90-4) with 2.17 wt %water with 2.30wt % of an aqueous solution of poly(methacrylic acid,sodium salt) (available from Sigma Aldrich UK CAS 54193-36-1) was madeup, and heated to 60-70° C., whilst stirring, to create a homogeneousmixture.

-   -   b) Starting from the anhydrous sodium acetate:

57.85 wt % sodium acetate (available from VWR International Ltd. assodium acetate anhydrous 99% CAS 127-09-3) with 39.84% water and 2.31 wt% aqueous solution of poly(methacrylic acid, sodium salt) was heated to60-70° C. whilst stirring, to create a homogeneous mixture.

To depress the melting and freezing temperatures, this solution wasadded in varying amounts with acetamide, as shown in the table above.Temperatures were recorded for the melt and freeze (nucleated with aseed crystal) of the samples, and the range of depressions is shownbelow. Crystallisation temperatures from 58° C. to 28.5° C. can begained by adding acetamide up to 70 molar %, whereupon the mix nears theeutectic point, after this point freezing temperature increases. This isshown in FIG. 3.

EXAMPLE 7 Addition of Trimethylolethane to Sodium Acetate Trihydrate toDepress Melting Point

Sodium acetate trihydrate solution was made as in the methods describedin Example 1. To make the mixture samples the SAT solution was added toTME as described in the table below.

Mass (g) Molar sodium acetate Ratio trihydrate TME 9:1 18.2132 1.78688:2 16.3836 3.6164 7:3 14.5096 5.4904 6:4 12.5895 7.4105

The mixtures can either be nucleated manually with a seed crystal, or2wt % of a nucleator, disodium hydrogen phosphate dihydrate. Byincreasing the content of trimethyolethane from 0 to 40 molar %, thefreezing point of the material has a range from 42° C. to 58° C.

The effect of increasing the amount of TME on the freezing temperatureof the samples Molar Temp/° C. Mass  0% 58   0% 10% 51.77  8.9% 20%49.74 18.1% 30% 46.96 27.5% 40% 42.49 37.1%

1-15. (canceled)
 16. A composition containing sodium acetate trihydrateas a phase change material comprising: (a) sodium acetate trihydrate orsodium acetate anhydrous; (b) one or more alkali soluble polymers; (c)one or more sodium acetate trihydrate nucleation promoters; and (d)water, wherein the one or more alkali soluble polymers acts as either acrystal habit modifier or a nucleation inhibitor or as a polymorphdirecting towards sodium acetate anhydrous resulting in abolition ofincongruent melting.
 17. The composition according to claim 16, whereinthe one or more alkali soluble polymers comprises: a copolymer ofmethacrylic acid and methyl methacrylate, or poly(methacrylic acidcomethylmethacrylate) or a sodium, potassium or zinc salt thereof; orpoly(methacrylic acid) or the sodium salt thereof; or a mixture thereof.18. The composition according to claim 16, wherein the sodium acetatetrihydrate is present in an amount of from about 80% to about 100%, byweight of the composition.
 19. The composition according to claim 16,wherein the anhydrous sodium acetate is present in an amount of fromabout 45% to about 60%, by weight of the composition.
 20. Thecomposition according to claim 16, wherein the one or morealkali-soluble polymers are independently present in an amount of fromabout 0.1% to about 10%, by weight of the composition.
 21. Thecomposition according to claim 16, wherein the one or more sodiumacetate trihydrate nucleation promoters are independently present in anamount of from about 0.1% to about 5%, by weight of the composition. 22.The composition according to claim 16, wherein the one or more sodiumacetate trihydrate nucleation promoters are independently selected fromthe group consisting of a metal phosphate, a metal pyrophosphate, ametal triphosphate, a metal polyphosphate, and mixtures and hydrogenacids thereof.
 23. The composition according to claim 16, wherein theone or more sodium acetate trihydrate nucleation promoters is disodiumhydrogen phosphate dihydrate and tetrasodium pyrophosphate decahydratepresent in an amount of from about 0.5% to about 2.5%.
 24. Thecomposition according to claim 16, wherein the wherein the one or morealkali soluble polymers is a polymethacrylic acid polymer having anaverage molecular weight in excess of 4,000.
 25. A process for preparinga phase change material according to claim 1, the process comprising thesteps of: a) combining sodium acetate trihydrate or sodium acetateanhydrous with the one or more alkali soluble polymers and, optionally,water to form a mixture; b) heating the mixture to above 58° C. to meltthe sodium acetate trihydrate until a homogenous liquid is obtained; andc) optionally, adding a nucleating promoter.
 26. The process accordingto claim 25, wherein the mixture is heated to 60° C. to 70° C.
 27. Theprocess according to claim 25, wherein the process is carried out in aheat battery or vessel.